1. Field of the Invention
The invention relates generally to method and apparatus for optical signal transmission. More particularly, this invention relates to bi-stable polarization control method and apparatus for configuring high-speed optical switches.
2. Descriptions of the Prior Art
As more fiber optic network systems are installed for carrying out optical signal transmissions, a technical challenge is still faced by those of ordinary skill to provide optical switches with high switching speed and long term operation reliability. An optical network system typically incorporates one or more switches to direct the optical paths for transmitting the optical signal to the desired destinations. In addition to the general requirements of low insertion loss, small cross talk, high extinction ratio, low polarization-dependent loss, etc., increasingly there is a demand for optical switches that have a high switching speed and good long-term reliability. The switching speed and reliability requirements are particularly important for optical network systems that demand high performance and long-term reliable signal transmissions.
Most optical switches implemented with prior art technology use mechanical switches, which utilize moving parts for controlling the optical signal transmission through different paths. Due to the need to mechanically move the optical element(s), switching speed is very limited, typically in millisecond range. Furthermore, the moving part is susceptible to material fatigue and worn out of linkages, particularly to these components connected to the moving parts. Long-term reliability becomes a major problem for design, operation, and maintenance of the optical network signal transmission systems as that discussed by P. G. Hale and R. Kompfner, in a paper xe2x80x9cMechanical Optical-Fiber Switch,xe2x80x9d Electron. Lett. 12, 388 (1976).
In order to overcome these difficulties, non-mechanical switches are implemented. The non-mechanical optical switches control the optical transmission paths of light by controlling the polarization state of a light by applying either magneto-optical or electro-optical control mechanism on the transmission of the light. In the case of using magneto-optic effect for controlling the switching operations, the typical device is composed of a soft Faraday rotator and an electromagnet. The magnetically soft Faraday rotator is located inside a cylindrical electromagnet that has coil windings around a soft magnet. Control of the magnetization state of the Faraday rotator is achieved by controlling the directions of the driving current in the coil. The drawback of this scheme is that it requires a continuous high current source to maintain the magnetization state in the Faraday rotator, resulting in high power consumption.
This problem can be alleviated by the more efficient, but sophisticated, electromagnet designs. Several prior art references discussed about these techniques. Specifically, in U.S. Pat. No. 5,048,937 entitled xe2x80x9cFaraday Rotator Device and Optical Switch Containing same,xe2x80x9d was issued on Sep. 17, 1991 to Shigeru Takeda and Satoshi Makio. An article entitled xe2x80x9cNon-mechanical optical switch for single-mode fibersxe2x80x9d was published in Applied Optics, Vol. 21, No.23, 4229-4234, 1982 by M. Shirasaki, H. Nakajima, T. Obokata, and K. Asama. Another article entitled xe2x80x9cMagneto-optical 2xc3x972 switch for single-mode fibers,xe2x80x9d was published in Applied Optics, Vol.23, No.19, 3272-3276, 1984, by M. Shirasaki, F. Wada, H. Takainatsu, H Nakajima, and K. Asama. Furthermore, a different electromagnet using semi-hard magnetic core material instead of the conventional soft magnets combining with a driving current pulse with finite time duration reduces the need for a continuous power supply. However, the material properties of the semi-hard magnet has to be carefully optimized so that it is not too hard to drive, yet hard enough to sustain the required remnant state. Specific details can be referred to U.S. Pat. No. 5,627,924, entitled xe2x80x9cArticle Comprising a non-mechanical optical fiber switch,xe2x80x9d issued on May 6, 1997 to S. Jin, I. Royer and T Tiefel. These devices however require complicated electromagnet design. Additionally, the devices are more expensive because sophisticated magnet with optimized material property has to be used.
Therefore, a need still exists in the art to provide a simple and compact switching device with high switching speed and long term reliability without requiring complicated electromagnets design and expensive materials such that these limitations and difficulties can be resolved. The key to the current invention is the utilization of a semi-hard or hard magneto-optical crystal in the Faraday rotator instead of the soft magneto-optical crystal used in the prior arts. This eliminates the need for both a continuous current source and various complicated electromagnets designs.
It is the object of the present invention to provide a compact, non-mechanical, non-blocking and high speed optical switch.
A preferred embodiment of this invention discloses a basic digital Faraday rotator device that comprises (a) a semi-hard or hard magneto-optic iron garnet-based crystal having bi-stable magnetization states at zero external magnetic field. (b) a wire winding around the crystal for changing the magnetization states by pulsed current having both fast rise time and short duration. (c) a circuit generating the required current pulses with both polarities. After a driving current pulse excitation to set the magnetization direction, the high coercive force and high remnant squareness in the garnet-based crystal will maintain the saturation magnetization state in the crystal without the need of any external current or magnetic field to sustain the remnant state.
The charging and discharging circuit through a capacitor controls the polarity of the driving current pulses and that controls the states of magnetization and polarization. The configuration significantly simplifies the circuit design. The current pulse is programmable and can also be controlled remotely.
The invention also includes several designs of 1xc3x972 and 2xc3x972 optical switches which can be constructed by different combinations of birefringent crystals, waveplates, GRIN lens, prisms, Wollaston prisms and the above mentioned digital Faraday rotator devices. The detailed optical schematics are described in the following sections.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed descriptions of the preferred embodiment that is illustrated in the various drawing figures.